1. Field of the Invention
The present disclosure relates generally to wireless communications, and more particularly to a method and apparatus for achieving power savings in a multiple-input multiple-output (MIMO) wireless receiver.
2. Related Art
In order to address the issue of increasing bandwidth requirements demanded for wireless communications systems, different schemes have been developed to allow multiple user terminals to communicate with a single access point by sharing the channel resources while achieving high data throughputs. Multiple Input Multiple Output (MIMO) technology represents one such approach that has recently emerged as a popular technique for next generation communication systems. MIMO technology has been adopted in several emerging wireless communications standards, including the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) using the additional dimensionalities created by the multiple transmit and receive antennas.
MIMO technology is used to improve both the throughput and the range of a WLAN system. However, such improvement comes with a cost of increased power requirements. Use of a multiple-chain radio transceiver increases the power consumption of a WLAN system for both transmit and receive modes. Moreover, transmissions can occur at any time, so a MIMO receiver must be prepared to receive (i.e., be listening for transmissions) at any time. In addition, a MIMO receiver must listen to the associated channel before allowing its corresponding MIMO transmitter to transmit. Maintaining all of the receiver chains of a multiple-chain transceiver in listen mode undesirably results in significant power consumption.
The IEEE 802.11n specification defines some power saving techniques for a MIMO system. However, these techniques suffer from several limitations. Thus, a MIMO station may use a single receive chain for a predetermined time period, thereby reducing power consumption of the MIMO station. However, the MIMO station must first notify an associated access point (AP) that the MIMO station is entering the single receive chain mode for the predetermined time period, thereby undesirably requiring involvement of the AP. Moreover, while the MIMO receiver is operating in the single receive chain mode, the MIMO receiver will use a single receive chain during both the listen mode and during an active receive (demodulation) mode. As a result, a MIMO receiver operating in the single receive chain mode will exhibit rate vs. range (RvR) performance loss, because gain associated with maximal ratio combining (MRC) is lost during the active receive mode. Note that the gain associated with MRC is only provided when signals from multiple receive chains are combined. In addition, the MIMO station is not able to support multi-stream (i.e., Ns greater than or equal to two)) transmit/receive operations while operating the single receive chain mode.
It would therefore be desirable to have a method and apparatus for overcoming the above-described problems. More specifically, it would be desirable to have a method and apparatus for reducing power consumption in a MIMO receiver that does not require the involvement of an associated AP, and does not result in (or reduces the chance of) loss of MIMO range and throughput benefits.